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Why Does NI 9411 Read 1.5V Between COM and the Digital Inputs?

Updated Jun 20, 2019

Reported In

Hardware

  • NI-9411

Issue Details

When I measure the voltage between the common line and a digital input on my NI 9411 digital input module I see roughly 1.5V. Why is this?

Solution

The 9411 doesn't have internal pull-up or pull-down resistors that pull the digital lines high or low. This being the case, the values on the pins float around 1.5V.

This allows you to add whichever resistor scheme you wish to have implemented in your measurement setup. However, adding pull-up or pull-down resistors is not necessary if the external device which is driving the digital lines can drive the signal both high and low.